Source:
National Center for Education Statistics, U.S. Dept of Education.About Student-Teacher Ratio

Student/teacher ratio is calculated by dividing the total number of students by the total number of full-time equivalent teachers. Please note that a smaller student/teacher ratio does not necessarily translate to smaller class size. In some instances, schools hire teachers part time, and some teachers are hired for specialized instruction with very small class sizes. These and other factors contribute to the student/teacher ratio. Note: For private schools, Student/teacher ratio may not include Pre-Kindergarten.